The invention described herein relates to nuclear reactors and more particularly to an emergency core cooling system which operates to supply coolant from an emergency source to the reactor core under conditions of reduction in pressure of primary coolant in the reactor.
The function of an emergency core cooling system for nuclear reactors is to immediately flood the reactor core with highly concentrated neutron absorber material which acts to terminate the fission process and simultaneously prevent heat damage to fuel and fuel rods in the event of a major rupture in the reactor primary coolant system piping.
The Atomic Energy Commission general design criteria for nuclear reactors requires that all operating reactors include an emergency core cooling system and although different designs have been developed for this purpose, one well known system utilizes large pressurized tanks or accumulators which discharge highly concentrated borated water directly into a pressurized water reactor when the pressure of primary coolant circulated from the reactor through a heat exchanger or steam generator drops below about 660 psi. This emergency coolant normally is injected into the primary coolant inlet pipes near the reactor inlet nozzles to assure delivery directly into the area containing the reactor fuel assemblies. It has been determined that current emergency cooling systems are completely effective to cool the fuel and fuel rods under circumstances of minor breaks in any component, including piping in the reactor coolant flow paths.
However, a major rupture, i.e., a full circumferential break and separation of the primary coolant inlet piping, creates unusual problems because the pressure of primary coolant still being circulated through the reactor from the other primary loops tends to cause the coolant to flow towards the area of reduced pressure which is represented by the pipe break. As the emergency coolant from the accumulator is then introduced into the inlet coolant pipes, the emergency coolant in-flow blocks the normal coolant attempting to escape through the fractured inlet pipe. Although the accumulator pressure is sufficient to overcome the out-flowing coolant, it is suspected that because of the tortuous inlet flow path in the reactor, the fuel and fuel rods comprising the core may become partially starved of coolant. Such starving and the accompanying drop in coolant pressure, allows the water circulating around the fuel rods to boil, thus generating bubbles which rise in the core and partially block flow laterally through the normal coolant outlet. Even though the likelihood or possibility of such a major rupture occurring is so extremely remote as to not dictate the need for designs to cover the situation, in view of the public interest, the systems are nevertheless designed to accommodate the most remote possibility of accidents.
In recognition of this problem, the emergency core cooling system disclosed in the above Frisch et al. patent application was designed to handle all loss of coolant situations by conducting emergency coolant downwardly through unused control rod guide thimbles or tubes into the reactor core. According to that disclosure, water enters the upper end of each guide thimble and is discharged near the lower end thereof inside the core in the form of a spray which is directed radially outward against adjacent fuel rods. This direct contact with the fuel rods thus increases the cooling efficiency and assures delivery of the required volume of water into the core. Tests on this type system show its great effectiveness, efficiency and reliability and the only known drawback lies in the labor and material costs necessary for installation and for subsequent removal when the reactor is being refueled.
The above discussion therefore suggests the need for a system which may independently be used for emergency cooling purposes, or alternatively, be used as a system supplemental to present emergency core cooling systems but of sufficient simplicity to eliminate the relatively high costs inherent in the Frisch et al design.